Re: [PATCH 03/10] writeback: Do not congestion sleep if there are no congested BDIs or significant writeback

[Mel Gorman <mel@csn.ul.ie>]

On Mon, Sep 13, 2010 at 12:37:44AM +0900, Minchan Kim wrote:
> > > > > > <SNIP>
> > > > > >
> > > > > > + * in sleeping but cond_resched() is called in case the current process has
> > > > > > + * consumed its CPU quota.
> > > > > > + */
> > > > > > +long wait_iff_congested(struct zone *zone, int sync, long timeout)
> > > > > > +{
> > > > > > +	long ret;
> > > > > > +	unsigned long start = jiffies;
> > > > > > +	DEFINE_WAIT(wait);
> > > > > > +	wait_queue_head_t *wqh = &congestion_wqh[sync];
> > > > > > +
> > > > > > +	/*
> > > > > > +	 * If there is no congestion, check the amount of writeback. If there
> > > > > > +	 * is no significant writeback and no congestion, just cond_resched
> > > > > > +	 */
> > > > > > +	if (atomic_read(&nr_bdi_congested[sync]) == 0) {
> > > > > > +		unsigned long inactive, writeback;
> > > > > > +
> > > > > > +		inactive = zone_page_state(zone, NR_INACTIVE_FILE) +
> > > > > > +				zone_page_state(zone, NR_INACTIVE_ANON);
> > > > > > +		writeback = zone_page_state(zone, NR_WRITEBACK);
> > > > > > +
> > > > > > +		/*
> > > > > > +		 * If less than half the inactive list is being written back,
> > > > > > +		 * reclaim might as well continue
> > > > > > +		 */
> > > > > > +		if (writeback < inactive / 2) {
> > > > > 
> > > > > I am not sure this is best.
> > > > > 
> > > > 
> > > > I'm not saying it is. The objective is to identify a situation where
> > > > sleeping until the next write or congestion clears is pointless. We have
> > > > already identified that we are not congested so the question is "are we
> > > > writing a lot at the moment?". The assumption is that if there is a lot
> > > > of writing going on, we might as well sleep until one completes rather
> > > > than reclaiming more.
> > > > 
> > > > This is the first effort at identifying pointless sleeps. Better ones
> > > > might be identified in the future but that shouldn't stop us making a
> > > > semi-sensible decision now.
> > > 
> > > nr_bdi_congested is no problem since we have used it for a long time.
> > > But you added new rule about writeback. 
> > > 
> > 
> > Yes, I'm trying to add a new rule about throttling in the page allocator
> > and from vmscan. As you can see from the results in the leader, we are
> > currently sleeping more than we need to.
> 
> I can see the about avoiding congestion_wait but can't find about 
> (writeback < incative / 2) hueristic result. 
> 

See the leader and each of the report sections entitled 
"FTrace Reclaim Statistics: congestion_wait". It provides a measure of
how sleep times are affected.

"congest waited" are waits due to calling congestion_wait. "conditional waited"
are those related to wait_iff_congested(). As you will see from the reports,
sleep times are reduced overall while callers of wait_iff_congested() still
go to sleep. The reports entitled "FTrace Reclaim Statistics: vmscan" show
how reclaim is behaving and indicators so far are that reclaim is not hurt
by introducing wait_iff_congested().

> > 
> > > Why I pointed out is that you added new rule and I hope let others know
> > > this change since they have a good idea or any opinions. 
> > > I think it's a one of roles as reviewer.
> > > 
> > 
> > Of course.
> > 
> > > > 
> > > > > 1. Without considering various speed class storage, could we fix it as half of inactive?
> > > > 
> > > > We don't really have a good means of identifying speed classes of
> > > > storage. Worse, we are considering on a zone-basis here, not a BDI
> > > > basis. The pages being written back in the zone could be backed by
> > > > anything so we cannot make decisions based on BDI speed.
> > > 
> > > True. So it's why I have below question.
> > > As you said, we don't have enough information in vmscan.
> > > So I am not sure how effective such semi-sensible decision is. 
> > > 
> > 
> > What additional metrics would you apply than the ones I used in the
> > leader mail?
> 
> effectiveness of (writeback < inactive / 2) heuristic. 
> 

Define effectiveness.

In the reports I gave, I reported on the sleep times and whether the full
timeout was slept or not. Sleep times are reduced while not negatively
impacting reclaim.

> > 
> > > I think best is to throttle in page-writeback well. 
> > 
> > I do not think there is a problem as such in page writeback throttling.
> > The problem is that we are going to sleep without any congestion or without
> > writes in progress. We sleep for a full timeout in this case for no reason
> > and this is what I'm trying to avoid.
> 
> Yes. I agree. 
> Just my concern is heuristic accuarcy I mentioned.
> In your previous verstion, you don't add the heuristic.

In the previous version, I also changed all callers to congestion_wait(). V1
simply was not that great a patch and Johannes pointed out that I wasn't
measuring the scanning/reclaim ratios to see how reclaim was impacted. The
reports now include this data and things are looking better.

> But suddenly you added it in this version. 
> So I think you have any clue to add it in this version.
> Please, write down cause and data if you have. 
> 

The leader has a large amount of data on how this and the other patches
affected results for a good variety of workloads.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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